Recently, with reduction in size of communication equipment, laminated dielectric filters effective for size reduction are used commonly as high-frequency filters. One example of conventional laminated dielectric filters is described with reference to drawings as follows.
FIG. 4 shows an exploded perspective view of a conventional laminated dielectric filter. The conventional laminated dielectric filter shown in FIG. 4 includes dielectric layers 301, shield electrodes 302a and 302b, resonator electrodes 303a, 303b, and 303c, capacitor electrodes 304a, 304b, 305a, 305b, 307a, 307b, and 307c, and side electrodes 308a, 308b, 308c, 308d, 309a, and 309b. In the dielectric layers 301, the shield electrode 302a, the resonator electrodes 303a, 303b, and 303c, the capacitor electrodes 304a, 304b, 305a, 305b, 307a, 307b, and 307c, and the shield electrode 302b are positioned sequentially. In addition, the side electrodes 308a and 308b on the left and right side faces of the dielectric body connect the shield electrodes 302a and 302b to form ground terminals. The side electrode 308c on the back face of the dielectric body connects the shield electrodes 302a and 302b and a common short-circuit end of the resonator electrodes 303a, 303b, and 303c to form a ground terminal. The side electrodes 308d on the front face of the dielectric body connect the capacitor electrodes 307a, 307b, and 307c corresponding to the open ends of the resonator electrodes 303a, 303b, and 303c, respectively. The side electrodes 309a and 309b on the left and right side faces of the dielectric body are connected to the capacitor electrodes 304a and 304b to form input/output terminals.
The structural view of the laminated dielectric filter thus configured is shown in FIGS. 5A and 5B. FIG. 5A is its left side view and FIG. 5B its front view. FIGS. 5A and 5B also show schematic capacitors formed between electrodes formed on an upper surface of a dielectric layer and electrodes formed on an upper surface of another dielectric layer, which oppose each other, respectively.
An equivalent circuit of the conventional laminated dielectric filter shown in FIGS. 4, 5A and 5B can be illustrated as shown in FIG. 6. The resonator electrodes 303a, 303b, and 303c form front end short-circuit ¼ wavelength resonators R303a, R303b, and R303c as shown in FIG. 6. The open ends of the resonators R303a, R303b, and R303c are connected to the ground terminals via the loading capacitor elements C307a, C307b, and C307c, respectively. The open ends of the resonators R303a and R303b are connected to each other via an inter-stage coupling capacitor element C305a and the open ends of the resonators R303b and R303c via an inter-stage coupling capacitor element C305b. Furthermore, the resonators R303a and R303c on the outer sides are connected to the input/output terminals via input/output coupling capacitor elements C304a and C304b, respectively.
Therefore, the laminated dielectric filter shown in FIG. 4 functions as a bandpass filter with the one ends of the capacitor elements C304a and C304b serving as the input/output ends. In addition, two attenuation poles are formed by a parallel resonance circuit formed of the inter-stage coupling capacitors C305a and C305b and magnetic-field couplings 401a and 401b occurring between the resonators R303a and R303b and between the resonators R303b and R303c. The frequencies of the attenuation poles depend on inter-stage coupling capacitance and the magnitude of the magnetic-field couplings, i.e. resonant gaps.
In the configuration as described above, however, the resonators R303a and R303c on the both sides bypass the resonator R303b positioned at the center to be coupled directly to each other by a magnetic-field coupling as indicated with the numeral 401c. Therefore, frequency characteristics of the two attenuation poles vary and thus the characteristics as designed cannot be obtained. The magnetic-field coupling 401c is determined uniquely when the magnetic-field couplings 401a and 401b are determined, i.e. when the resonant gaps are determined. Consequently, the two attenuation poles cannot be controlled freely while consideration is given to the magnetic-field coupling 401c. 